Method for pulling a single crystal

ABSTRACT

A method and apparatus for pulling a single crystal by, for example, the Czochralski method where a melt is heated by heaters arranged around, or around and below a crucible. Auxiliary heaters are provided above the crucible to directly heat the melt to supplement the heaters arranged around, or around and below the crucible, so as to reduce the power of the heaters arranged around, or around and below the crucible. According to the method, a single crystal is pulled under the condition of the local highest temperature of a quartz crucible of 1600° C. or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus for pulling asingle crystal and, more particularly, a method and an apparatus forpulling a single crystal wherein a single crystal of silicon or the likeis pulled by the Czochralski method (hereinafter, referred to as CZmethod) and the like.

2. Description of the Relevant Art

As a silicon single crystal ingot used for manufacturing a substrate forforming a circuit component of a LSI (large scale integrated circuit)and the like, a silicon single crystal pulled by the CZ method has beenmainly used. FIG. 3 is a diagrammatic sectional view of an apparatus forpulling a single crystal used for the CZ method, and in the figure,reference numeral 11 represents a crucible.

The crucible 11 comprises a bottomed cylindrical quartz crucible 11a anda bottomed cylindrical graphite crucible 11b fitted on the outer side ofthe quartz crucible 11a and supporting the quartz crucible 11a. Thecrucible 11 is supported with a support shaft 18 which rotates in thedirection shown by the arrow in the figure at a prescribed speed. Acylindrical heater 12a of a resistance heating type and a perforateddiscoidal heater 12b are arranged around and below the crucible 11,respectively. A heat insulating mould 17 is concentrically arrangedaround the heater 12a. The crucible 11 is charged with a melt 13 of amaterial for forming a crystal which is melted by the heater 12a. On thecentral axis of the crucible 11, a pulling axis 14 made of a pulling rodor wire is suspended, and at the lower end part thereof, a seed crystal15 is held by a seed chuck 14a. Each part mentioned above is arranged ata fixed place in a water cooled type chamber 19 wherein pressure andtemperature can be controlled.

In pulling a single crystal 26, the pressure in the chamber 19 isreduced and kept in the reduced state for an appropriate period of timeso as to sufficiently release a gas contained in the melt 13. Then aninert gas is introduced into the chamber from the upper part, so as tomake an inert gas atmosphere under reduced pressure within the chamber.In the step of seeding, the seed crystal 15 held at the lower end of thepulling axis 14 is brought into contact with the surface of the melt 13,so as to slightly melt the front portion of the seed crystal 15 melt. Inthe next step of necking, while the pulling axis 14 is rotated on thesame axis in the reverse direction of the support shaft 18 at aprescribed speed, a crystal (neck) having a diameter narrowed down to2-4 mm and a length of 20-40 mm is made to grow from the lower endportion of the seed crystal 15. After the necking step, the diameter ofthe single crystal 26 is gradually increased to form a shoulder having aprescribed diameter, and then, a main body is pulled which forms to aproduct.

In the last stage of the pulling, in order to prevent inducement of highdensity dislocation from being caused by a steep temperature gradient,the diameter of the single crystal 26 is decreased, leading to theformation of an end-cone and a tail end, and then, the single crystal 26is separated from the melt 13. Cooling the single crystal 26 is at theend of the pulling. As described above, pulling a single crystal 26comprises a great number of steps, and in general, it takes several tensof hours from melting a material for a crystal to the completion ofpulling a single crystal 26.

Recently, in order to achieve more efficient production of a singlecrystal ingot and improvement in the yield of chips, the diameter of apulled single crystal ingot has gradually become larger and the quantityof material for a single crystal required at one pulling has also becomelarger. Accordingly, the diameter of the crucible to make the materialfor a single crystal melt therein has also become larger, the requiredpower of a heater has also become higher, and the time required forpulling a single crystal has also become longer. Consequently, a quartzcrucible in direct contact with the melt has been exposed to a hightemperature for a long period of time, resulting in more and moreserious deterioration thereof. The deterioration of the quartz crucibleallows bubbles entrained therein to grow, and when the bubbles arereleased, the melt is contaminated with quartz particles. When thequartz particles become incorporated into a single crystal at thecrystal growth interface, the occurrence rate of dislocation increases.

SUMMARY OF THE INVENTION

The present invention was developed in order to solve the abovedescribed problems, and it is an object of the present invention toprovide a method and an apparatus for pulling a single crystal, by whichdeterioration of a quartz crucible caused by exposure to a hightemperature for a long period of time can be prevented, so thatoccurrence of dislocation caused by the crucible deterioration can beinhibited.

In order to achieve the above object, the method for pulling a singlecrystal according to the present invention, wherein a single crystal ispulled by dipping a seed crystal into a melt formed in a quartzcrucible, is characterized by having the condition of the local highesttemperature of the quartz crucible of 1600° C. or less.

With the above method for pulling a single crystal, deterioration of thequartz crucible can be prevented and an occurrence rate of dislocationin a single crystal caused by the crucible deterioration can be helddown.

The apparatus for pulling a single crystal according to the presentinvention, which is used for the above method for pulling a singlecrystal, is characterized by having heaters for heating a melt in aquartz crucible above and around, or above, below and around the quartzcrucible.

Using the above apparatus for pulling a single crystal, a melt can bedirectly heated by the heaters arranged above the quartz crucible, sothat a local rise in crucible temperature caused by occurrence oftemperature unevenness of the quartz crucible can be prevented. As aresult, deterioration of the quartz crucible can be prevented, the lifespan thereof can be lengthened, and occurrence of dislocation in asingle crystal can be inhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sectional view showing an apparatus for pullinga single crystal according to the present invention;

FIG. 2 is a graph indicating the relationship between the occurrencerate of dislocation in a single crystal and the local highesttemperature of the quartz crucible in the Examples and in a ComparativeExample; and

FIG. 3 is a diagrammatic sectional view showing an apparatus for pullinga single crystal used for the conventional CZ method.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of a method and an apparatus for pulling a singlecrystal according to the present invention are described below byreference to those Figures.

FIG. 1 is a diagrammatic sectional view of an apparatus for pulling asingle crystal according to the embodiments of the invention, and theapparatus is similar to the conventional apparatus for pulling a singlecrystal shown in FIG. 3 except for auxiliary heaters 20 arranged above acrucible 11. Accordingly, here, the description of the composition ofthe other parts of the apparatus for pulling a single crystal accordingto the embodiments is omitted and the auxiliary heater 20 is described.

The auxiliary heater 20, comprising an infrared lamp 21 and a mirror 22,the lamp having associated internals leads 23a and external leads 23b,can directly heat the melt 13 by gathering within a quartz crucible 11ainfrared radiation emitted from the infrared lamp 21 by means of themirror 22. The infrared lamp 21 and the mirror 22 fixed on the upperwall of a chamber 19 are required to have heat resistance. As a materialfor the mirror 22, a heat-resistant metal such as W and Mo ispreferable. A bulb and a heating element constituting the infrared lamp21 are preferably made of vitreous silica or the like, and tungsten orthe like, respectively. It is advantageous that the auxiliary heater 20is located as close to the melt 13 as possible, but taking intoconsideration the heat resisting properties of the materialsconstituting the auxiliary heater 20, it is desirable that the auxiliaryheater 20 be located about 200 mm above the melt surface.

Here, in the present embodiments, an infrared lamp 21 forming part ofthe auxiliary heater 20 is exemplified, but in other embodiments, acarbon heater, laser generator, electron beam generator, or highfrequency heater and so on may be used as part of the auxiliary heater20.

A method for pulling a single crystal according to the embodiments ofthe present invention is described below.

After a crucible 11 is charged with a material for forming a crystal,heaters 12a and 12b arranged around and below the crucible 11 are turnedon to heat and melt the material for forming a crystal in the crucible11. In this case, too, using auxiliary heaters 20 makes it possible tomelt the material for forming a crystal more rapidly in the quartzcrucible 11a having a lowered highest temperature.

In addition, in pulling a single crystal 16, irradiating the surface ofthe melt 13 with an infrared radiation using the auxiliary heaters 20and directly heating the melt 13 make it possible to decrease power ofthe heaters 12a and 12b arranged around, or around and below, thecrucible 11 and keep the hottest part of the quartz crucible 11a at1600° C. or less.

Here, a method for pulling a single crystal 16 by the CZ method isdescribed, but in a method known as melting method wherein a moltenlayer and a solid layer are formed as the upper part and the lower partwithin the quartz crucible 11a, respectively, to pull a single crystal16, using auxiliary heaters 20 according to the invention enables anexcessive rise in local temperature of the quartz crucible 11a to beinhibited, resulting in inhibition of the occurrence of dislocation inthe single crystal 16.

EXAMPLES

Examples and a Comparative Example of a method and an apparatus forpulling a single crystal according to the present invention aredescribed below. In each Example, using the apparatus for pulling asingle crystal shown in FIG. 1, a silicon single crystal 16 having adiameter of 6 inches and a length of 500 mm was produced. The conditionsare shown in the Tables below. As a Comparative Example, the data of thecase wherein a silicon single crystal 26 was produced in the same mannerusing the conventional apparatus for pulling a single crystal shown inFIG. 3 are also shown in the Tables below.

                  TABLE 1    ______________________________________    Apparatus for pulling a single crystal used in the present    ______________________________________    invention    Type of heater 12a                    Resistance heating-type    Dimentions of heater 12a                    Inside diameter: 730 mm                    Outside diameter: 780 mm                    Thickness: 25 mm    Auxiliary heater 20                    Combination of infrared lamp 21 and                    mirror 22 (two heaters)    Vertical distance between                    170 mm    upper rim of crucible 11    and auxiliary heater 20    Shape of mirror 22                    Circle in plane figure                    (diameter: 200 mm)    Dimensions of quartz                    Diameter: 610 mm    crucible 11a    Depth: 360 mm    Dimensions of chamber 19                    Inside diameter: 1200 mm                    Height: 1880 mm    Atmosphere in chamber 19                    Ar atmosphere    Flow of Ar      60 liters per minute    Pressure        1.33 × 10.sup.3 Pa    ______________________________________     Prepared quantity of material: 150 kg

                  TABLE 2    ______________________________________    Melting conditions of material                               Time              Power of heater (kW)                               for              Heater                    Heater    Auxiliary                                       melting              12a   12b       heater 20                                       (Hr)    ______________________________________    Examples 1-3                50      40        40      8    Comparative 70      40        --     10    Example 1    (no auxiliary heater)    ______________________________________

                  TABLE 3    ______________________________________    Pulling conditions                             Highest    Power of            Power of heater (kW)                             temperature                                        Time for    heater or            Heater  Heater  Auxiliary                                   of quartz                                            pulling    else    12a     12b     heater 20                                   crucible (° C.)                                            (Hr)    ______________________________________    Example 1            50      0       40     1500     28    Example 2            59      0       32     1550     28    Example 3            64      0       25     1600     28    Comparative            70      0       0      1650     30    Example 1    ______________________________________     Note) The temperature of the quartz crucible 11a was measured with a     number of thermocouples inserted between the quartz crucible 11a and the     graphite crucible 11b where rotation of the crucible 11 was stopped.

                  TABLE 4    ______________________________________            Rotation speed          Rotation speed    Other   of single    Pulling speed                                    of crucible    conditions            crystal (rpm)                         (mm/min)   (rpm)    ______________________________________    Example 1            12           0.7        6    Example 2            12           0.7        6    Example 3            12           0.7        6    Comparative            12           0.7        6    Example 1    ______________________________________

FIG. 2 is a graph indicating the relationship between the occurrencerate of dislocation in the silicon single crystal 16 or 26 and the localhighest temperature of the quartz crucible 11a according to Examples 1-3and Comparative Example 1.

As obvious from FIG. 2 in Examples 1-3, each occurrence rate ofdislocation was less than 5%, while in the conventional method(comparative Example 1), the occurrence rate of dislocation steeplyincreased to about 8% when the local highest temperature of the quartzcrucible 11a exceeded 1600° C.

As described above, using the apparatus for pulling a single crystalaccording to the present invention enables the local highest temperatureof the quartz crucible 11a to be 1600° C. or less, leading to theprevention of the deterioration of the quartz crucible 11a. As result,the occurrence rate of dislocation in the pulled single crystal 16 canbe held to less than 5%.

What is claimed is:
 1. A method of pulling a single crystal, comprisingthe steps of dipping a seed crystal in a metal formed in a quartzcrucible and pulling a single crystal, wherein the single crystal ispulled under a condition of a local highest temperature of the quartzcrucible is 1600° C. or less.